Wcs: Warn when encountering non-standard FITS WCS keywords

This will emit Python "FITSFixedWarning"s when wcslib encounters non-standard FITS WCS keywords (that it knows about).

This requires some changes to wcslib itself to get it to work, but doesn't break compatibility with upstream wcslib.  If the user builds with the system "stock" wcslib, they will see the warnings printing to stderr, but not converted to Python warnings.

I will submit the diff upstream to Mark Calabretta -- he has been amenable to this kind of change (to better extract error messages) in the past.